[Haskell-beginners] Re: [Haskell-cafe] Just how unsafe is unsafe

Jake McArthur jake at pikewerks.com
Thu Feb 5 16:36:51 EST 2009

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Andrew Wagner wrote:
| I understand that unsafeXXX allows impurity, which defiles our ability
| to reason logically about haskell programs like we would like to. My
| question is, to what extent is this true?

My opinion is that unsafeXXX is acceptable only when its use is
preserved behind an abstraction that is referentially transparent and
type safe. Others may be able to help refine this statement.

| Suppose we had a module, UnsafeRandoms, which had a function that would
| allow you to generate a different random number every time you call it.
| The semantics are relatively well-defined, impurity is safely sectioned
| off in its own impure module, which is clearly labeled as such. How much
| damage does this do?

This does not preserve referential transparency, so by my criteria above
this is an unacceptable use of an unsafe function. One reason it's a bad
idea is that it removes determinism, which may be very important for

- - Jake
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